A model for the study of the feasibility of nonoperative, pharmacologic manipulation of the flow in a systemic-to-pulmonary shunt is produced by constructing a right subclavian-to-pulmonary shunt in acutely prepared canines. The appropriate measurements of flows and pressures are recorded so that total systemic, and pulmonary resistances might be calculated simultaneously with recorded shunt flows. Such baseline data are recorded after volume infusion has been employed to attain slightly elevated filling pressures in the heart. Afterload reduction is then instituted by sodium nitroprusside infusion and increased incrementally while pressures and flows are constantly recorded, until the increase in afterload reduction are continued until arterial pressures fall below acceptable limits or until significant reduction in shunt flow is noted.